The invention relates to surge arresters and other types of electrical power distribution equipment.
Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
Upon completion of the over-voltage condition, the surge arrester returns to operation in the high impedance mode. This prevents normal current at the system frequency from following the surge current to ground along the current path through the surge arrester.
Conventional surge arresters typically include an elongated outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and electrical ground, and an array of other electrical components that form a series electrical path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high resistance when exposed to a normal operating voltage, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to varistors, a surge arrester also may include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the varistors. Some arresters also include electrically-conductive spacer elements coaxially aligned with the varistors and gap assemblies.
For proper arrester operation, contact must be maintained between the components of the stack. To accomplish this, it is known to apply an axial load to the elements of the stack. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the elements, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the elements and the end terminals.
One way to apply this load is to employ springs within the housing to urge the stacked elements into engagement with one another. Another way to apply the load is to wrap the stack of arrester elements with glass fibers so as to axially-compress the elements within the stack.
In one general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. The Young""s modulus of the joint is less than approximately half that of the Young""s modulus of the electrical components.
Embodiments of the joint may include one or more of the following features. For example, the Young""s modulus of the joint may be approximately one-eightieth to one-tenth of the Young""s modulus of the electrical components. More particularly, the Young""s modulus of the joint may be approximately one fortieth of the Young""s modulus of the electrical components. The Young""s modulus of the joint may be between approximately 200,000 psi and 1,600,000 psi and the Young""s modulus of the electrical components may be between approximately 13,000,000 psi and 18,000,000 psi. More particularly, the Young""s modulus of the joint may be between approximately 300,000 psi and 500,000 psi and the Young""s modulus of the electrical components may be between approximately 14,000,000 psi and 17,000,000 psi. Even more particularly, the Young""s modulus of the joint may be approximately 400,000 psi and the Young""s modulus of the electrical components may be approximately 15,000,000 psi.
The joint creates a region between the electrical components that is mechanically more compliant than the components themselves. One reason for the greater compliance within the joint is a Young""s modulus which can be less than half of that of the electrical components. The lower modulus of the joint serves to attenuate or dampen the thermo-mechanical forces generated within the electrical components during operation of, for example, a surge arrester.
The joint may further include an electrically conductive polymer that provides mechanical compliance. The joint also may further include an electrically-conductive, mechanically-compliant metal alloy.
The joint may be between approximately one-sixteenth of an inch thick and one-half of an inch thick. More particularly, the joint may be between approximately one-eighth to three-eighths of an inch thick. Even more particularly, the joint may be approximately one-fourth of an inch thick. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity is provided by a first layer of an electrically-conductive adhesive adhered to the lower face of the first electrical component and a second layer of the electrically-conductive adhesive adhered to the upper face of the second electrical component. Mechanical compliance is provided by the two layers of electrically-conductive adhesive and by a polymer composite layer that is between the two layers.
Embodiments of the joint may include one or more of the following features. For example, the joint may further include a conductive shunt having a first end, a second end, and a middle section connecting the first end and the second end. The first end is positioned in the first layer of electrically-conductive adhesive, the second end is positioned in the second layer of electrically-conductive adhesive, and the middle section passes through the polymer composite layer.
The polymer composite layer may include a first surface in contact with the first layer of adhesive, a second surface in contact with the second layer of adhesive, a first opening on the first surface, a second opening on the second surface, and a channel passing between the first and second openings. The conductive adhesive of the first and second layers also is in the channel so that it provides an electrically-conductive path between the first and second layers. The polymer composite layer may further include multiple channels passing between multiple first and second openings, with the conductive adhesive in the multiple channels so that they provide electrically-conductive paths between the first and second layers. The polymer composite layer may be electrically-conductive to provide a continuous current path. The polymer composite layer also may be electrically nonconductive, in which case the conductive shunt conducts electricity between the electrical components. The joint may be incorporated in an electrical device.
In another implementation, the polymer composite layer may include an electrically-conductive material. The electrically-conductive material may include a silver alloy and/or a carbon black filler. The polymer composite layer may include an electrically-conductive composite of metal and polymer. The metal may include copper, aluminum or brass, or combinations of those metals. The polymer composite layer also may be a polymer layer having a metal coating on its surfaces. The polymer composite layer may further include electrically-conductive pins passing through the polymer layer and terminating in the adhesive layer. The opposite ends of the conductive pins may be adhered to the upper face of the second electrical component and the lower face of the first electrical component.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity is provided by a first region or layer of an electrically-conductive adhesive adhered to the lower face, a second region or layer of the electrically-conductive adhesive adhered to the upper face, and a metal layer disposed between the first layer and the second layer. Mechanical compliance is provided by the two layers of electrically-conductive adhesive and by the metal layer.
Embodiments of the joint may include one or more of the following features. For example, the metal layer may include a foam metal plate that may be made from aluminum or another metal. The foam metal plate may be a porous metal structure impregnated with an epoxy or an adhesive. The metal layer also may be a deformable metal plate having a first surface defining peaks and valleys and a second opposite facing surface defining peaks and valleys, and may be made from aluminum or copper, or a combination of those and/or other metals. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity is provided by regions or layers of an electrically-conductive adhesive, and mechanical compliance is provided by alternating regions or layers of an electrically nonconductive adhesive. The electrical apparatus may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity is provided by at least one metal spring adhered on a first end to the upper face and on a second end to the lower face. Mechanical compliance is provided by the metal spring and an adhesive positioned between the upper face and the lower face.
Embodiments of the joint may include one or more of the following features. For example, the adhesive may be an electrically-conductive adhesive that provides electrical conductivity for the joint. A nonconductive adhesive also may be used instead of the electrically-conductive adhesive. The joint may further include additional metal springs adhered on each first end to the upper face of the second electrical component and on each second end to the lower face of the first electrical component. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity and mechanical compliance are provided by an electrically-conductive shunt having a first end, a second end, and a middle section that connects the first and second ends, a first solder joint that electrically connects the first end to the lower face of the first electrical component, and a second solder joint that electrically connects the second end to the upper face of the second electrical component.
Embodiments of the joint may include one or more of the following features. For example, further mechanical compliance may be provided by an epoxy layer positioned between the lower face and the upper face, and surrounding the electrically-conductive shunt. The shunt may be a spring or multiple springs. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Mechanical compliance is provided by alternating regions or layers of a metal and an epoxy. Electrical compliance is provided by the regions or layers of metal. The alternating regions or layers of the metal and the epoxy are oriented perpendicularly to the upper surface and the lower surface.
Embodiments of the joint may include one or more of the following features. For example, the regions of metal may be regions or layers of a foam metal. The regions or layers of the foam metal may include a porous structure impregnated with an epoxy, an elastomer, or an adhesive, or a combination of these. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity is provided by at least one metal wire adhered on a first end to the upper face of the second electrical component and on a second end to the lower face of the first electrical component. Mechanical compliance is provided by the metal wire and an adhesive layer positioned between the upper face and the lower face.
Embodiments of the joint may include one or more of the following features. For example, the metal wire may be adhered to the upper face and the lower face by solder. Additional metal wires may be adhered on each first end to the upper face and on each second end to the lower face. The metal wire may be in the form of one or more springs or one or more straps. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Mechanical compliance and electrical conductivity are provided by a first layer of solder adhered to the lower face of the first electrical component, a second layer of solder adhered to the upper face of the second electrical component, and a metal layer between the first and second layers of solder.
Embodiments of the joint may include one or more of the following features. For example, the metal layer also may be a deformable metal plate having a first surface defining peaks and valleys and a second opposite facing surface defining peaks and valleys, and may be made from aluminum or copper, or a combination of those metals. The metal layer also may be a foam metal plate, and the foam metal plate may be a porous structure impregnated with an epoxy or an adhesive. The metal layer also may be a polymer composite layer having a surface metallization of a conductive material. The polymer composite may include a carbon black or a silver alloy filler. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Mechanical compliance and electrical conductivity are provided by an electrically-conductive adherent layer and a multi-component structure.
Embodiments of the joint may include one or more of the following features. For example, the electrically-conductive adherent layer may be an epoxy, a conductive adhesive, or a solder, or a combination of these materials. The joint may be incorporated in an electrical device.
The multi-component structure may include a pair of opposing metal plates, a first outer O-ring positioned between the opposing metal plates, and a second outer O-ring positioned around the first outer O-ring and between the opposing metal plates. A first adhesive is disposed between the opposing metal plates in a space defined between an inner diameter of the second outer O-ring and an outer diameter of the first O-ring. A second adhesive is disposed between the opposing metal plates in a spaced defined by the inner diameter of the first O-ring. The first adhesive may be an electrically nonconductive adhesive and the second adhesive may be an electrically-conductive adhesive.
The multi-component structure may further include a pair of opposing metal plates and a nonconductive layer. Each metal plate may have at least one conductive projection projecting in the direction of the opposing metal plate and being conductively connected to the opposing conductive projection. The nonconductive layer may be positioned between the opposing metal plates and encapsulate the conductive projections. The conductive projections may be connected by a metal wire, or may be directly connected. The nonconductive layer may be, for example, a nonconductive adhesive, a nonconductive epoxy, or a nonconductive polymer composite, or a combination of these materials.
The electrically-conductive and mechanically-compliant joint between a pair of electrical components provides numerous advantages. For example, mechanical compliance may serve to attenuate the mechanical forces generated throughout the electrical components during operation. In this manner, the service duration of the device in which the electrical components are installed will be lengthened because the attenuated forces are less likely to harm the electrical components and joints. The use of nonconductive materials, along with conductive materials, in the joint, may reduce the overall cost of the joint. The electrical conductivity and mechanical compliance of the joint may be tailored by varying the ratio of the quantity of materials used in the joint.
Other features and advantages of the invention will be apparent from the description and drawings, and from the claims.